This invention relates to the fabrication of solid state crystal or microelectronic devices such as surface acoustic wave (SAW) devices. Such SAW devices may be used as high-Q resonators or as delay lines. In SAW devices the acoustic waves travel only along one surface of the crystal substrate, and thus high frequency operation can be achieved with a relatively thick crystal substrate, which is typically quartz. In bulk acoustic wave devices in which the entire crystal structure vibrates, high frequency operation can be achieved only by reducing the crystal thickness. Such thin crystals are fragile and subject to flexing in use from such environmental factors as shock and acceleration.
A known type of SAW resonator comprises a relatively thick quartz substrate with a pair of interdigital transducers (IDTs) located near the center of one of the broad surfaces of the substrate, with a grooved array reflector at each end of the same substrate surface, symmetrically located with respect to the two IDTs. The grooved arrays are efficient reflectors of surface acoustic waves for a band of frequencies determined by the groove spacing and groove depth. The IDTs are relatively wideband and are used for coupling to the electricl circuit. The electrical signal applied to the metallic IDTs strains the quartz substrate to generate the surface acoustic waves. The IDTs comprise an interdigital pattern of conductors with an overlap pattern chosen to couple energy only to the fundamental mode of the high-Q Fabry-Perot cavity formed by the two grooved array reflectors. The spacing of the grooves in the arrays thus determines the resonant frequency. The reflector arrays each comprise a plurality of parallel grooves which are usually formed in the substrate by etching away material between an array of parallel strips of masking material. These masking strips may be aluminum which is formed on the substrate by photolithography techniques. After the grooves are etched the aluminum strips are usually removed. The IDTs are usually formed during the same photolithography process by which the aluminum masking strips are formed, and thus it is necessary to somehow mask the IDTs from the etching step which forms the grooves of the reflector arrays. In the prior art, the IDTs have been covered with a resist material which shielded the IDTs from the bombarding ions which were used to etch the grooves of the reflector arrays. After this etching step in the prior art, the resist material had to be removed. A description of this prior art method appears on pages 138-139 of the book "Precision Frequency Control", edited by Gerber and Ballato, Academic Press, Inc., 1985. The present invention comprises an improvement of this fabrication method which makes the use of this masking resist material unnecessary.